In recent years, in error correcting codes in wireless communications, attention has been focused on turbo codes nearing the Shannon limit that is theoretical limit of transmission speed enabling transmission without errors. In turbo coding, generally, a plurality of bit sequences is output such as a sequence of systematic bits that are information bits, a sequence of parity bits that are redundant bits obtained by performing convolutional coding on the systematic bits, and another sequence of parity bits obtained by performing interleaving on the systematic bits and then convolutional coding thereon.
A technique for transmitting thus output plural bit sequences from respective plural transmission antennas is called space-time turbo coding. Space-time turbo coding is one of SDM (Space Division Multiplexing) that spatially multiplexes a plurality of signals on the transmission side.
In space-time turbo coding, a plurality of bit sequences generated by turbo coding is modulated, subjected to symbol mapping, and transmitted from a plurality of transmission antennas assigned to respective bit sequences. For example, see IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 49, No. 1, JANUARY 2001 “Space-Time Turbo Codes with Full Antenna Diversity”, and IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 19, NO. 5, MAY 2001 “Turbo-Coded Modulation for Systems with Transmit and Receive Antenna Diversity over Block Fading Channels: System Model, Decoding Approaches, and Practical Considerations”.
By this means, it is possible to achieve both the diversity gain due to transmission using a plurality of transmission antennas and significant coding gain by turbo codes.
However, in performing space-time turbo coding, using M-ary modulation to further improve the transmission efficiency causes such a problem that a large number of overlaps of signal points (hereinafter, referred to as “degeneracy”) occur in symbol mapping.
This problem will specifically be described below with reference to FIGS. 1A to 1C. FIGS. 1A to 1C are diagrams showing signal point constellations when a plurality of bit sequences obtained by turbo coding is transmitted from two transmission antennas, as an example of space-time turbo coding.
FIG. 1A is a diagram showing one example of signal point constellations when BPSK (Binary Phase Shift Keying) modulation is performed on each of the bit sequences transmitted from two transmission antennas.
When each of BPSK-modulated bit sequences is subjected to symbol mapping and multiplexed, originally four (=2×2) signal point candidates should exist. However, in the case as shown in FIG. 1A, a white point shown in the figure represents overlapping two points among four points, and degeneracy occurs.
Similarly, FIG. 1B is a diagram showing one example of signal point constellations when QPSK (Quadrature Phase Shift Keying) modulation is performed on each of the bit sequences transmitted from two transmission antennas.
When each of QPSK-modulated bit sequences is subjected to symbol mapping and multiplexed, originally sixteen (=4×4) signal point candidates should exist. However, in the case as shown in FIG. 1B, points except four black points shown in the figure represent overlapping two or four points, and degeneracy occurs. In other words, in the case of QPSK modulation, degeneracy occurs in 75% of entire points, and the probability of occurrence of degeneracy is higher than that in BPSK modulation.
Further, FIG. 1C is a diagram showing one example of signal point constellations when 16QAM (16 Quadrature Amplitude Modulation) is performed on each of the bit sequences transmitted from two transmission antennas.
When each of 16QAM-modulated bit sequences is subjected to symbol mapping and multiplexed, originally 256 (=16×16) signal point candidates should exist. However, in the case as shown in FIG. 1C, points except four black points shown in the figure represent overlapping two or four points, and degeneracy occurs. In other words, in 16QAM, degeneracy occurs in 98% of entire points, and the probability of occurrence of degeneracy is higher than that in QPSK modulation.
Thus, in performing M-ary modulation in space-time turbo coding, since a large amount of degeneracy occurs, demodulation performance deteriorates in a reception apparatus. In order to prevent the demodulation performance from deteriorating, it is necessary to increase transmission power in a transmission apparatus to enhance reception quality in the reception apparatus. As a result, however, increased interfering power is provided to reception apparatuses other than the transmission destination, and adverse effects are exerted on the entire wireless communication system.